1. Field of the Invention
The present invention relates to a ridge waveguide type semiconductor laser device, and more specifically, to a ridge waveguide type semiconductor laser device capable of maintaining stable fundamental transverse mode operation without involving any jumps of a longitudinal oscillation mode, even with use of high optical output of 300 mW or more.
2. Prior Art
A semiconductor laser device that is built up on a GaAs substrate and oscillates in a 980-nm band can be effectively used as a pumping laser device for an Er-doped fiber amplifier (EDFA). An optical output at the output end of an Er-doped fiber that is coupled to this laser device is expected to be at a value near 100 mW.
If the efficiency of coupling between the Er-doped fiber and the laser device is 60%, realization of the aforesaid fiber end output requires the optical output of a laser beam from the laser device to be at 170 mW or thereabout.
If a semiconductor laser device is used as the laser device, moreover, it is expected to perform stable fundamental transverse mode oscillation in order to ensure satisfactory efficiency of coupling between the laser device and a single-mode fiber. If the semiconductor laser device undergoes high-order mode oscillation, its optical output increases. In this case, however, the coupling efficiency of the laser beam and the single-mode fiber worsens extremely, so that the optical output at the output end of the fiber lowers.
Thus, if the semiconductor laser device that is coupled to the fiber is injected with current to be actuated, the upper limit of the injected current level at which the laser device can maintain the fundamental transverse mode operation is an important problem in increasing the optical output at the output end of the fiber.
In general, a ridge waveguide type semiconductor laser device is used as the semiconductor laser device that oscillates in the 980-nm band. In this laser device, AlGaAs is used as the material of its cladding layers and the top portion of its upper cladding layer is formed as a ridge waveguide to ensure high operation reliability.
For the ridge waveguide type semiconductor laser device constructed in this manner, theoretical calculations for necessary conditions for the fundamental transverse mode operation of the laser device have been published (H. Jaeckel et al., IEEE J. Quantum Electron, vol. 27, No. 6, pp. 1,560-1,567, 1991).
Proposed in the above report is a ridge waveguide type semiconductor laser device of the layer structure shown in FIG. 1. This device has a stacked structure that is composed of a substrate 1 of n-GaAs and several layers formed thereon through the buffer layer. These layers include a lower cladding layer 2 of n-Al0.3Ga0.7As, a lower optical confinement layer 3 of n-AlGaAs, an active layer 4 of a strain type quantum well structure, formed of InGaAs/GaAs, an upper optical confinement layer 5 of p-AlGaAs, and an upper cladding layer 6 of p-Al0.3Ga0.7As, which are built up in the order named. The top portion of the upper cladding layer 6 constitutes a ridge waveguide 7, and a contact layer 8 of p-GaAs is formed on the top surface of the waveguide 7. The other portion of the upper cladding layer 6 is covered with an insulating protective film 9 of Si3N4. An upper electrode 10 of Tixe2x80x94Ptxe2x80x94Au is formed on the top surface of the protective film 9, and a lower electrode 11 of Gexe2x80x94Auxe2x80x94Ni is formed on the back surface of the substrate 1.
In obtaining the necessary conditions for the fundamental transverse mode operation of the semiconductor laser device, according to the aforementioned report, the aforesaid stacked structure is converted into a layer structure model in which the active layer 4 is interposed between the AlGaAs layers, as shown in FIG. 2. The relation between the width (W) of a base portion 7a of the ridge waveguide 7 and the distance (t) from the bottom ridge portion 7a to the active layer 4 is theoretically calculated for the case where the structure model shows fundamental transverse mode oscillation in the relationship between the single mode fiber. FIG. 3 shows the result of the calculation.
According to the aforementioned report, the obtained semiconductor laser device can perform the fundamental transverse mode operation under any conditions if the width (W) of the base portion 7a of the ridge to be formed and the distance (t) from the bottom ridge portion 7a to the active layer 4 are adjusted individually to values that fulfill a hatched region S0 in FIG. 3, in manufacturing the ridge waveguide type semiconductor laser device.
In the case of the ridge waveguide type semiconductor laser device designed in accordance with the aforesaid theoretical calculations, the fundamental transverse mode operation can be realized with high yield if the optical output of the laser beam ranges from 200 to 250 mW.
If the injected current is increased to obtain a higher optical output, however, it is hard to maintain stable fundamental transverse mode operation, and the oscillation wavelength starts unstable behavior in response to the instability of the transverse mode.
In the case of a semiconductor laser device that was manufactured with the width (W) of the bottom ridge portion 7a at 3.2 xcexcm and the distance (t) from the bottom ridge portion 7a to the active layer at 0.3 xcexcm, according to an experiment conducted by the inventors hereof, for example, the fundamental transverse mode operation was kept stable without involving any jumps of the longitudinal mode (oscillation wavelength) when the injected current was not higher than about 280 mA (about 250 mW in terms of optical output). When the injected current was made higher, however, the fundamental transverse mode operation became unstable.
Thus, it was ascertained that a ridge waveguide type semiconductor laser device that oscillates in the 980-nm band, which requires high optical output, could not be designed in accordance with a W-t relational diagram that appears in the aforementioned report.
The object of the present invention is to provide a novel ridge waveguide type semiconductor laser device, which can solve the aforementioned problems of the conventional ridge waveguide type semiconductor laser device, and in which design parameters of a ridge waveguide are set in the following manner so that fundamental transverse mode operation can be maintained even with use of high optical output of 300 mW or more, and the longitudinal mode (oscillation wavelength) is restrained from jumping.
In order to achieve the above object, the inventors hereof varied the width (W) of a bottom ridge portion of a ridge waveguide type semiconductor laser device to be formed and the distance (t) from the bottom ridge portion to an active layer, and measured kink current for each case to examine the stability of fundamental transverse mode operation. In consequence, it was newly found that if a ridge waveguide is designed with values W and t (mentioned later) in a region off the hatched region S0 in FIG. 3 in the aforementioned report, the kink current in the semiconductor laser device is as high as about 400 mA, and the longitudinal mode (oscillation wavelength) can be stabilized.
Based on this knowledge, the inventors hereof further studied and developed a ridge waveguide type semiconductor laser device of the present invention.
Thus, according to the present invention, there is provided a ridge waveguide type semiconductor laser device comprising a stacked structure composed of a substrate and layers thereon, including a lower cladding layer, lower optical confinement layer, active layer, upper optical confinement layer, and upper cladding layer built up in the order named, the upper cladding layer having an upper part in the shape of a ridge and a lower part situated on the active layer, and the width of the bottom ridge portion of the ridge ranging from 2.5 xcexcm to 5.0 xcexcm and the distance from the bottom ridge to the active layer being adjusted to a value higher than 0.5 xcexcm and not higher than 0.8 xcexcm.